This invention relates to a color overhead projector which reads an image from a color original, writes the image into a film and illuminates the film to project the image on a screen.
The inventors formerly proposed an overhead projector capable of writing into a thermochromic film an image read out from an original, illuminating the film to project the image on a screen (Japanese Patent Application No. 61(1986)-309,334).
The thermochromic film possesses such a property that it becomes transparent on being heated to a first temperature, for example, about 60.degree. C. and retains its transparence even after being cooled to normal room temperature, becomes opaque on being heated to a second temperature, for example, 72.degree. C., and retains its opacity even after being cooled to normal temperature. When the film on which an image has been already formed is exposed uniformly to the first temperature (transparentizing temperature), therefor, the image is erased. When the film which has been transparentized to erase the old image is exposed selectively to the second temperature in conformity with an original image, a new image is formed on it. Owing to this peculiar property, the same thermochromic film can be used repeatedly. The projector of the aforementioned proposal (Japanese Patent Application No. 61(1986)-309,334) is capable of recording the image read out from an original, namely printing the image on a thermographic recording paper with a thermal head, besides writing the image in the thermochromic film with the same thermal head. The projector of the aforementioned proposal is so constructed that the thermal head is switched between the thermochromic film side and the thermographic paper side. The proper pressure of the thermal head to be exerted on the thermochromic film is in the range of 200 to 400 g/cm.sup.2 and that to be exerted on the thermographic paper is in the range of 3 to 5 kg/cm.sup.2.
In the projector of the aforementioned proposal, however, the pressure of the thermal head which is produced by a head pressing means is constant and cannot be changed to be suitable for the case where the thermal head is used on the thermochromic film and the case where it is used on the thermographic recording paper. Therefore it is difficult for the projector to produce clear images on both the thermochromic film and the thermographic recording paper equally.
The thermochromic film is produced by forming a thermochromic layer on one surface of a base film of polyester, for example. The thermochromic layer softens when the thermochromic film is heated over the state transition temperature (that is, softening temperature). Therefore, in the device of the aforementioned proposal which is so constructed that the thermochromic layer, for the erasure of an old image, is heated by being pressed against a heat roller, the softening of the thermochromic layer entails the disadvantages that the film suffers flaws and impressions on its surface by the heat roller, and the roller is smeared with the softened thermochromic layer. The quality of the image on the screen produced by the device therefore, is very inferior due to above described disadvantages. Further, when the thermochromic layer which has been heated to the transparentizing temperature is suddenly cooled by contact with a conveying roller, for example, before it is cooled naturally to a temperature below the softening point, it opacifies to such an extent that erasing the old image becomes practically impossible.